The present invention relates to the technical field of the treatment of exhaust gases, in particular the treatment of nitrogen oxide-containing exhaust gases.
The present invention relates in particular to a method for the treatment of nitrogen oxide-containing exhaust gases from technical processes, such as flue gases, for the purposes of removing and/or separating off the nitrogen oxides and/or for the purposes of reducing the nitrogen oxide content by means of chemical reduction of the nitrogen oxides. In particular, the present invention relates to a method for the denitrification of exhaust gases from large technical installations, such as for example power plants, in particular combined heat and power plants, or waste incineration installations.
Furthermore, the present invention relates to an apparatus for the treatment of nitrogen oxide-containing exhaust gases from technical processes, such as flue gases, for the purposes of removing and/or separating off the nitrogen oxides and/or for the purposes of reducing the nitrogen oxide content by means of chemical reduction of the nitrogen oxides.
Furthermore, the present invention relates to the use of an apparatus for removing and/or separating off nitrogen oxides from nitrogen oxide-containing exhaust gases from technical processes, in particular flue gases, and to the use of an apparatus for the selective cooling of flue gases from technical processes, in particular flue gases.
Finally, the present invention relates to a method for the treatment of exhaust gases from technical processes, in particular flue gases, preferably for the purposes of cooling the gases, in particular as part of a method for removing and/or separating off nitrogen oxides from exhaust gases from technical processes.
In the case of combustion reactions in the presence of air, metastable, generally poisonous and reactive oxides of nitrogen, so-called nitrogen oxides, are formed. The formation of nitrogen oxides is intensified by the combustion or thermolysis and pyrolysis of organic and inorganic nitrogen-containing compounds, such as occurs in large combustion installations such as combined heat and power plants or waste incineration installations.
Nitrogen oxides, in particular the compounds nitrogen monoxide and nitrogen dioxide, which are known under the designation “nitrous gases” and which are also referred to for short as NOx, are however not only poisonous and lead to irritation and damage to the respiratory organs, but also promote the formation of acid rain, as they react with moisture to form acids.
The release of nitrogen oxides is however also a problem for additional environmental protection reasons, as they firstly promote the formation of smog and hazardous ground-level ozone and secondly, as greenhouse gases, intensify global warming.
Owing to the disadvantageous effects of nitrogen oxides on health and on the environment, and not least owing to the associated economic damage, it has already long been attempted to minimize or prevent the release of nitrogen oxides from combustion processes. In passenger motor vehicles, this is achieved for example through the use of catalytic converters, which permit an almost complete removal of the nitrogen oxides from the exhaust gases.
To reduce the nitrogen oxide emissions from large technical installations, in particular from large industrial combustion installations, taking into consideration the respective legal situation and business considerations, various methods for denitrogenization or denitrification (DeNOx) have been developed which, on their own or in combination, are intended to yield an effective reduction or elimination of nitrogen oxides in exhaust gases, in particular flue gases.
Methods or measures for reducing the nitrogen oxide content of exhaust gases, in particular flue gases, can in this case be divided into primary and secondary measures:
In the case of the primary measures, the combustion process is controlled such that the nitrogen oxide content in the resulting exhaust gases is as low as possible; it is the intention, in effect, that the nitrogen oxides should not arise in the first place. Primary measures include, for example, flue gas recirculation, wherein the flue gas is conducted again into the combustion zone, and air and fuel stages, wherein the combustion is controlled such that different combustion zones with different oxygen concentrations are realized. Furthermore, the formation of nitrogen oxides in flue gases can also be reduced through the addition of additives or by quenching, that is to say by the injection of water for the purpose of lowering the temperature during the combustion process.
By contrast to the primary measures, which are intended to prevent the formation of nitrogen oxides, the use of the secondary measures is intended to reduce the concentration of the nitrogen oxides in the exhaust gases, in particular flue gases. Secondary measures include for example separation methods, with which the nitrogen oxides are chemically bound or scrubbed out of the flue-gas flow. A disadvantage of the separation methods is however that large amounts of waste products are generated, such as for example process water, which are often contaminated with further flue gas constituents and must be disposed of in a costly manner.
Therefore, in modern large technical installations, as secondary measures, use is normally made of methods which are based on a reduction of the nitrogen oxides to form elementary nitrogen and which produce only small amounts of waste substances, wherein a distinction is generally made between catalytic and non-catalytic methods.
The selective catalytic reduction (SCR) of nitrogen oxides encompasses catalytic methods in which the nitrogen oxides are converted to form elementary nitrogen with the aid of metal catalysts. With SCR methods, it is generally possible to attain the best denitrification values, although the use of the catalyst makes the method considerably more expensive and less economically viable. Furthermore, installations for carrying out the SCR method are extremely expensive not only in terms of purchase but also in terms of maintenance, as the sensitive catalytic converters must undergo maintenance, or be replaced, at short time intervals. Specifically in the case of large combustion installations in which the fuel composition can often be determined only with inadequate accuracy, such as for example waste incineration installations, there is therefore always the risk of poisoning of the catalytic converters by contaminants in the flue gas. This risk can be reduced only through the implementation of additional expensive measures.
By contrast, selective non-catalytic reduction (SNCR) is based on the thermolysis of nitrogen compounds, in particular ammonia or urea, which then react with the nitrogen oxides in a comproportionation reaction to form elementary nitrogen.
Selective non-catalytic reduction can be carried out at considerably lower cost than selective catalytic reduction: the costs for the purchasing and maintenance of SNCR installations amount to just 10 to 20% of the costs of corresponding SCR installations.
A problem of the SNCR method is however that the effectiveness thereof does not come close to matching the effectiveness of catalytic methods, such that, for example in the event of a further reduction of the legally permitted limit values for nitrogen oxides in exhaust gases, in particular flue gases, most SNCR installations would no longer be allowed to continue operating.
A further disadvantage of the methods based on the selective non-catalytic reduction of nitrogen oxides is that excess reducing agent must be used, which reducing agent does not react completely, such that the exhaust gas contains a certain and in some cases not insignificant amount of ammonia. Excess ammonia in the exhaust gas must either be separated off, or reduced in terms of content by process engineering measures, so as to enable the exhaust-gas flow to be released to the environment.
Furthermore, there are also methods which are based both on a catalytic effect and on the use of reducing agents, though in the case of these methods, too, the main disadvantages of the respective methods (high costs for the use of catalytic methods, and low effectiveness in the case of the use of reducing agents) cannot be overcome.
Although, of late, novel SNCR installations have been developed which are based on the combined use of multiple reducing agents and which exhibit effectiveness virtually equivalent to that of catalytic methods, such installations however cannot provide optimum results under all operating conditions.
In particular in the case of combustion boilers being retrofitted with SNCR installations and during the operation of combustion boilers under full load, it is often the case that an injection of the reducing agent in a temperature range expedient for the SNCR method is not possible owing to the design of the boiler, or the temperatures that are expedient for the reduction are attained in the region of the heating surfaces or heat exchangers. In these cases, a major part of the flue gases, which may amount to up to 50% of the flue gas volume, cannot be reached by the reducing agent, or the reducing agent must be introduced into the exhaust-gas flow in an unfavorable temperature range. Furthermore, with the use of urea as reducing agent, there is, in the region of the heat exchangers, the risk of deposition of ammonia or ammonium salts and thus of corrosion.
The introduction or injection of the reducing agent into the flue-gas flow in the optimum temperature range is however critical for the effectiveness of the nitrogen oxide reduction and thus of the denitrification.
With the injection of the reducing agent above 1100° C., the reducing agent is increasingly oxidized to form nitrogen oxides, whereby firstly the nitrogen oxide rate of separation decreases and, secondly, the consumption of reducing agent increases. By contrast, if injection is performed at excessively low temperatures, the reaction rate decreases, whereby so-called ammonia slippage occurs which, over the further path travelled by the exhaust gases, in particular by the flue gases, leads to the formation of ammonia or ammonium salts. This gives rise to secondary problems such as, for example, contamination of the fly ash with ammonia or ammonium salts, the amount of which is considerably increased, and the disposal of which is also made more cumbersome and thus more expensive.
It has already been attempted to counteract this problem for example by arranging the heating surfaces or heat exchangers in movable fashion above the combustion boiler or injecting reducing agents by means of relatively long water-cooled injection lances. These modifications however also cannot prevent the occurrence of operationally induced temperature gradients, that is to say large temperature differences and different flows speeds in a plane perpendicular to the main flow direction of the exhaust gases, which have the effect that the reducing agents are not distributed uniformly over the entire boiler cross section. It is thus always the case that reducing agents are injected into flue gas regions which lie outside the effective temperature window or range. This in turn results in an inadequate degree of nitrogen oxide separation, high reducing agent consumption, and a high level of ammonia slippage.
To counteract these problems, tests have already been carried out in which water was additionally admixed to the reducing agents in order to cool the flue gases to the temperatures required for reduction. With this method, it is possible in individual cases to achieve improvements in the separating-off or removal of nitrogen oxides from the flue gases, wherein, however, the execution of the method overall remained unsatisfactory.
Accordingly, in the presence of low load in the combustion boiler and low flue gas temperatures, it would be necessary to reduce the amount of water in order to prevent excessive cooling of the flue gases. Since, during the injection, the droplet spectrum and thus the penetration depth and the distribution of the reducing agents in the exhaust-gas flow are dependent on the respective throughflow rate, an excessively low water quantity leads to an unfavorable distribution of the reducing agents in the flue gas and thus to a decreasing degree of separation of the nitrogen oxides and to increased consumption of reducing agents.
By contrast, in the case of falling flue gas temperatures, if the liquid quantity is kept constant, the droplet distribution or the droplet spectrum and thus the penetration depth and the distribution of the reducing agents in flue gases remain constant, but the flue gas is cooled to such an extent that the reduction conditions are unfavorable, and increased ammonia slippage results.
Finally, said method also does not solve the problem that, in many boiler installations, in particular ones equipped with a denitrification installation, the temperature window for effective and optimum reduction of the nitrogen oxides contained in the flue gases, in particular at full load of the boiler installation, often prevails in the poorly accessible narrow intermediate spaces in the region of the heat exchangers or heating surfaces. In said region, it is firstly difficult to actually achieve the entire flue gas volume with the reducing agent, and secondly, in particular with the use of urea, it cannot be ruled out that urea particles will impinge on the boiler tubes and heat exchangers or heating surfaces and lead to corrosion damage there. The use of urea solutions as reducing agent however cannot always be avoided owing to the greater penetration depth into the flue gas.
Overall, therefore, it is evident that, until now, there has not yet been developed an inexpensive and flexible method for the denitrification of exhaust gases from technical processes, such as flue gases, which method constantly permits good rates of separation of nitrogen oxides from exhaust gases from technical processes even in the case of adverse or unfavorable structural designs of the combustion boiler.